In general, screw type fasteners are unitary devices formed with a head at one end of an elongated shaft. The shaft is threaded over all or a portion of its length. One form has its shaft tapered to smaller diameter in the direction away from the head at least in the region of its distal end. The other form has a threaded shaft of uniform diameter.
Screw type fasteners are used to secure one element to another. Ordinarily, the shaft of the fastener extends freely through a hole in one element, whereas the threads of the shaft engage the material of the second element. Rotation of the fastener screws its threads into the second element until further rotation is prevented upon abutment of the fastener head against the surface of the first element. The tapered fastener is used when the material of the second element is relatively soft. In that case, an unthreaded pilot hole may be formed in the second member. The fastener enters the pilot hole and is driven along it. As the fastener is turned, the material of the second element is deformed to the shape of mating threads. Such a fastener is called a wood screw and is used for fastening element to wood and to readily deformable plastics.
Screw fasteners intended for securing things to metal usually have straight cylindrical, threaded shafts. The metal, the second element or nut, into which the fastener is to be turned is formed with an internally threaded hole to receive the threads of the fastener. When the head is formed with a kerf or a slot to receive a twisting tool, such as a screw driver or nut driver or an Allen wrench, the fastener is called a machine screw. Larger sizes of such fasteners are arranged to be twisted by a wrench and are called bolts.
A variation of the tapered shaft, threaded fastener called a sheet metal screw, is formed with deep thread grooves, often pitched at about the thickness of two metal sheets. Designed for use in holding two sheet metal pieces together, the sheet metal screw is inserted through a hole larger than the shaft in one sheet metal element into a smaller hole in a second sheet metal element. The holes are sized, relative to the screw shank diameter and thread pitch such that the screw cams the two sheets together with or without an upsetting of the periphery of the hole in the second member.
There are many variations in the form and in the manner in which such screw fasteners are used. In one of the most important of these variations, no pilot hole, or only a small pilot hole, is formed. Sheet metal screws especially are made in that form and are called "self tapping."
The installation and removal of these threaded fasteners is accomplished by rotating them about the axis of the shaft with a screw driver or nut driver or, in one special case, an Allen wrench. In each case, the screw fastener and its installation and removal tool are formed with complementary conformations which permit application of torque to the fastener. Four forms of interfitting conformations are common. Most common is the kerf, a slot formed diametrically across the outer face of the fastener's head, which is driven by a blade which ideally is almost as wide as the kerf is long. Next most common is the Phillip's conformation. The driver has a conical end the side wall of which is grooved along its length at four regions equally spaced around its circumference. A depression having the inverse of that shape is formed in the head of the screw fastener.
Cap screws and set screws are found in machinery and in metal working tooling. They are often formed with a straight walled hexagonal depression in the head which accommodates an Allen wrench. The fourth form employs the hexagonal head of bolts. It is normally driven with a nut driver, a tool with a hexgonally shaped depression in its working end.
The three primary considerations in selecting head form are cost of the fastener, the physical qualities of the substance into which the fastener is to be driven, and convenience and cost considerations involved in selection and use of the driving tool.
Maximum leverage for turning the fastener is available when its head has hexagonal form. However, the fact that the hexagonal head (1) cannot be recessed, (2) is aesthetically less pleasing than other head forms, and (3) requires a driving tool that closely matches the head in size, accounts for limited use except in the interior and hidden regions of mass produced products.
The hexagonal head presents another serious problem. The driver must be aligned rather exactly with the head before the driver and head can be mated. That problem is overcome in some degree in the Phillips configuration. A small degree of lateral and axial misalignment does not prevent mating of the driver and head. It is as acceptable, or even more so, than the kerfed head, and for those reasons is widely used in commercial products.
But the Phillips configuration is not widely used in applications where the fastener might be removed in affecting repairs and the like. Because the bearing surfaces against which the driver must act are close to the axis of the head, mechanical advantage is small and large force is required to affect rotation. The conformations are easily damaged unless the driver and head are closely aligned and the driver size closely matches the head cavity.
Mechanical purchase almost equal to that of the hexagonal head is available in the kerfed head and blade combination, when the width of the driver approaches the length of the kerf. An advantage is that the size of the driver and kerf need not be closely matched. However, the driver must be aligned rotationally, axially, and laterally to enable proper mating of driver blade and kerf. Consequently, the kerfed head, while most popular and practical for small volume tasks and where ease of removal is important, is generally unsuited for mass production use.